The present invention generally relates to a microelectronic tunable capacitor and a method for fabrication, and more particularly, relates to a microelectronic tunable capacitor that can be fabricated by micro-electro-mechanical-system (MEMS) technology compatible with a CMOS process and a MEMS method for fabricating the capacitor.
Miniaturization of motors, actuators and similar machine parts is receiving increasing attention because of the new uses of these devices made possible because of their small size. Additionally, these devices can be manufactured in large quantities at low piece-part cost. Current designs of miniaturized machine parts can be categorized according to size or scale. Macroscopic machine parts have a length in the range of approximately 1 to 10 inches, and while microscopic machine parts, sometimes referred to as MEMS (Micro-Electro-Mechanical-Systems) have a length in the range of 0.01 to 1 inch.
In any event, existing miniaturized actuators and motors of both macroscopic or microscopic size are essentially replicas of larger motors, and thus include such component parts as windings, stators, gears, transmission links, etc. These miniaturized parts must be assembled with high precision in order to produce an operable device providing the desired function, e.g. movement of an electrically activated component that then mechanically engages other parts to induce motion. Depending upon the engagement configuration, this motion may be linear in any of several axes, rotary, circular, etc. Because of the number of complex parts that must be assembled with a high degree of precision, the yields of parts meeting target specifications and performance are relatively low using current manufacturing processes. These low yields in turn increase the cost of the parts. Accordingly, it would be desirable to provide a new form of actuator and related method for inducing movement of an object on a microscopic or macroscopic scale which eliminates the problems mentioned above.
The MEMS technology has recently been extended to the semiconductor fabrication industry. In the present state of the art, a semiconductor device is normally formed in a planar structure and therefore the process for fabricating the semiconductor device is generally a planar process. For instance, layers of different materials, i.e. such as insulating materials and metallic conducting materials, are deposited on top of one another and then features of the device are etched through the various layers. The planar fabrication process, while adequate in fabricating most semiconductor elements and devices, is not suitable for fabricating certain devices that are 3-dimensional in nature. For instance, a 3-D solenoid, i.e. or a 3-D inductor coil, must be fabricated by stacking a large number of layers from the bottom to the top and therefore, requires a large number of photomasks to complete the task. For instance, when CMOS technology is used in forming such 3-D solenoid, at least four other steps utilizing photomasks must be incorporated in order to complete the fabrication process. Moreover, the precise alignment between the layers is necessary in order to avoid a variety of processing difficulties occurring at the interfaces.
In recent years, there have been great advances in the application of semiconductor fabrication technology applied to the wireless communication industry. In wireless communication, the use of a microelectronic capacitor or a microelectronic tunable capacitor is an important aspect of the technology. For instance, tunable matching networks, electronically tunable filters and voltage-controlled oscillators have been widely used in microwave communications. To be successfully used in such applications, a tunable capacitor must have a high Q-factor and a wide adjustable range. The wide adjustable range not only provides the necessary frequency range, but also compensates process or temperature induced variations.
Tunable capacitors fabricated by the presently available semiconductor fabrication technology are able to meet the high Q-factor, however, the available tunable range and the possibility of process integration with the standard IC fabrication process are less than desirable. While efforts have been made in designing tunable capacitors by changing the distance between two capacitor plates, the design has many limitations. Among them, the pull-in effect greatly limits the tunable range of the capacitance. In another design that utilizes comb drive-type actuators by changing the overlapped area between two parallel capacitor plates, the amount of displacement between the two parallel plates is limited and thus, impossible to produce the desirable tunable range. Furthermore, the complicated fabrication process of the comb drive structure further limits the potential for process integration with standard IC fabrication technology.
It is therefore an object of the present invention to provide a microelectronic tunable capacitor that does not have the drawbacks or shortcomings of the conventional tunable capacitors.
It is another object of the present invention to provide a microelectronic tunable capacitor that can be fabricated by a MEMS technology.
It is a further object of the present invention to provide a microelectronic tunable capacitor that can be fabricated on a semiconductor substrate by a standard CMOS technology.
It is another further object of the present invention to provide a microelectronic tunable capacitor fabricated by MEMS technology which can be integrated with standard IC fabrication process.
It is still another object of the present invention to provide a microelectronic tunable capacitor which has a capacitive capable of being changed by varying the overlapped area between two capacitor plates without the pull-in defect.
It is yet another object of the present invention to provide a microelectronic tunable capacitor that can be tuned in a broad capacitance range.
It is still another further object of the present invention to provide a MEMS method for fabricating a microelectronic tunable capacitor.
It is yet another further object of the present invention to provide a method for fabricating a microelectronic tunable capacitor by integrating a MEMS technology and a standard CMOS process.
In accordance with the present invention, a microelectronic tunable capacitor and a method for fabricating the capacitor are provided.
In a preferred embodiment, a microelectronic tunable capacitor is provided which includes a micro-actuator formed by a pair of fixed electrodes positioned spaced-apart from each other sandwiching a suspended arm electrode swayable between the pair of fixed electrodes, the suspended arm electrode has a polarity that is opposite to a polarity of the pair of fixed electrodes; a first capacitor plate situated in a fixed position that has a predetermined area; and a second capacitor plate having an area substantially similar to the predetermined area of the first capacitor plate mounted to a tip of the suspended arm electrode for swaying by an electrostatic force between the pair of fixed electrodes from a completely overlapped position between the first and second capacitor plates achieving maximum capacitance to a completely non-overlapped position achieving minimum capacitance.
In the microelectronic tunable capacitor, the pair of fixed electrodes each has a curvilinear shape curving away outwardly from the suspended arm electrode. The suspended arm electrode is mounted to a fixed position base allowing the electrode to sway from side-to-side in-between the pair of fixed electrodes. The curvilinear shape has a curvilinear surface coated with an insulating material for stopping the movement of the suspended arm electrode. The insulating material used may be silicon oxide. The curvilinear shape may have a curvilinear surface covered with a conductive metal layer for stopping the movement of the suspended arm electrode, wherein the suspended arm electrode may be formed of AlCu or W. The first and the second capacitor plates may be formed of Al, Cu or AlCu. The pair of fixed electrodes may be formed of AlCu or W. The pair of fixed electrodes may have a negative polarity, while the suspended arm electrode may have a positive polarity.
The present invention is further directed to a method for fabricating a microelectronic tunable capacitor that can be carried out by the operating steps of providing a pre-processed semiconductor substrate that includes a bulk silicon substrate; a first dielectric material layer on top of the bulk silicon substrate; at least four metal layers with three dielectric material layers there-in-between formed on top of the first dielectric material layer; depositing a silicon oxide layer on top of and encasing the at least four metal layers; depositing a photoresist layer on top of the silicon oxide layer; patterning and etching an opening through the silicon oxide layer at a first end of the pre-processed semiconductor substrate exposing a second to the uppermost metal layer; etching away the second to the uppermost metal layer forming a structure for a first capacitor plate; and etching away partially a silicon oxide layer on top of and below a metal layer immediately under the second to the uppermost metal layer forming the first capacitor plate situated on a first silicon oxide support on the silicon substrate and a second capacitor plate situated on a second silicon oxide support on the silicon substrate.
The method for fabricating a microelectronic tunable capacitor may further include the step of forming the at least four metal layers with AlCu. The method may further include the step of etching away the second to the uppermost metal layer by a wet etching technique, or by a wet etchant that has a high selectivity toward silicon oxide. The wet etchant may include H3PO4, HNO3, CH3COOH and H2O2. The method may further include the step of etching away partially the silicon oxide layer by a dry etching method, or by a dry etchant that has high selectivity toward the at least four metal layers. The dry etchant may include F or Cl.